1. Field of the Invention
2. Brief Description of the Prior Art
Valve regulated lead acid batteries (VRLA) were introduced in the late 1980's as “maintenance free.” In this type of battery, oxygen and hydrogen produced during electrochemical reactions in the battery recombine to maintain an aqueous liquid electrolyte at a constant level with the cell. As a result, these batteries have only a small amount of liquid electrolyte. Discharge of a VRLA battery module to a current compensated voltage of less than substantially 1.5 volts significantly increases the likelihood of irreversible conversion of the active battery material, lead oxide, to lead sulfate due to pinch-off or isolation effects. A drop in capacity proportional to the damage subsequently results.
When VRLA batteries are being charged, they often suffer a charge deficit that cumulatively increases with each charge. The amount of charge deficit varies from battery to battery and those with a smaller deficit are referred to as “more receptive” to charging current and those with a large deficit are “less receptive.” One way of compensating for the charge deficit of a battery is to increase the voltage to which the battery is charged, i.e. the “float” voltage. When the voltage of a battery reaches a manufacturer specified float-voltage, it is deemed fully charged. However, increasing the float-voltage to remedy the charge deficit of a less receptive battery can overcharge those batteries in the string that are more receptive. Overcharging causes disassociation of the electrolyte and consequent gas pressurization in a VRLA battery. If the pressure exceeds the relief valve setting, gas escapes and electrolyte is lost, with permanent loss of capacity as the result. The mismatch in charge receptivity grows with the number of charge cycles. When one battery in the string finally suffers an unacceptable loss of capacity, all of the batteries in the string must usually be discarded, although many of them have substantial useful life remaining.